This invention relates to an X-ray tube.
As shown in FIG. 1, a known X-ray tube of large capacity having a rotary anode comprises an evacuated envelope 1, a cathode unit 2 disposed in the envelope 1, and an anode unit 3 disposed in the envelope 1 to face the cathode unit 2. The evacuated envelope 1 consists of end portions 1a and 1c made of glass and an intermediate portion 1b made of metal. The anode unit 3 includes a target 4 facing the cathode unit 2 and a rotor 6 for rotating the target 4. The X-ray tube further comprises a stator 7 for rotating the rotor 6 and an insulation hollow cylinder 8 for insulating the rotor 6 from the stator 7. The cathode unit 2 has a cup 9 containing a filament for emitting an electron beam.
An electron beam from the filament hits the target 4, thereby generating X-rays. The X-rays are emitted outside through a window 10 provided on the evacuated envelope 1. As X-rays are generated, the target 4 is heated to a high temperature. The heat of the target 4 mostly radiates from the surface of the target 4 and partly is transmitted to the rotor 6 through a shaft 5 connecting target 4 to the rotor 6. As a result, the rotor 6 is heated mostly by the heat radiating from the target 4 and partly by the heat transmitted via the shaft 5. As it is heated more and more, the rotor 6 operates less efficiently for the following reasons.
As shown in FIG. 2, the rotor 6 comprises a rotor-cylinder 11, a shaft 12 extending in the rotor-cylinder 11 and attached at the upper end to the rotor-cylinder 11 by means of a screw, a pair of bearings 13 provided the upper and lower end portions of the shaft 12, respectively, and a support 14 disposed in the rotor-cylinder 11 and surrounding the bearings 13. As mentioned above, the heat of the target 4 mostly radiates to the rotor 6 and partly is transmitted to the rotor 6 via the shaft 5. As the rotor 6 is heated gradually, so is the shaft 12 in the rotor-cylinder 11. Ultimately, the bearings 13 are heated gradually, too. The heat of the bearings 13 is transmitted to the support 14, and it is emitted outside the rotor 6. Here occurs a temperature difference between the inner and outer races of each bearing 13. Generally, a uniform clearance of a few microns is provided between the races and ball of bearing 13 to achieve a smooth rotation of the rotor 6. A smooth rotation of the rotor 6 would be impossible if the bearings 13 thermally expand due to the temperature difference between the inner and outer races of the bearings 13.
Further, the known X-ray tube of FIG. 1 is defective in the following respect. When impinged with an electron beam, the target 4 emits secondary electrons. The secondary electrons hit the evacuated envelope 1 so hard that the end portions of the envelope 1, both made of glass, are broken in some cases.